This invention relates to diesel exhaust particulate traps, and more particularly to a side entry or cross-flow trap employing a plurality of trap elements and to the ceramic trap elements themselves.
Particulate traps designed to fulfill the requirements of diesel engines necessarily are required to meet a number of conflicting parameters. It is important that the trap elements have a sufficiently small pore size for the effective trapping of the diesel exhaust particulates, and at the same time present a low pressure so that the trap does not material exceed the back pressure of a conventional muffler. Further, the trap elements must possess a substantial filtering surface or capture volume, so that the entire trap can be fabricated within an envelope which does not substantially exceed that of a conventional muffler. Also, the trap must be susceptible to ease of regeneration.
Diesel particulates traps for filtering and trapping particulate material, such as a soot, from the exhaust of diesel engines, have generally been constructed as a monolithic honeycomb. Such traps commonly have a multitude of internal parallel chambers separated by thin porous internal membranes or walls. Part of such chambers are dead-ended or end sealed, to force the exhaust gas to traverse across the porous wall before exiting.
Trap regeneration typically is accomplished by heating the trap to a temperature sufficiently high to burn off the entrapped particles. While regeneration is taking place, it is usually necessary to bypass the trap, or to provide a second parallel connected trap for use during regeneration.
The component parts making up the trap, including the operative internal portions, must be sufficiently rugged to withstand use, and even abuse, over a long service life. The trapping material and the trap elements made of such material must be able to withstand mechanical and thermal shocks, and must be adequately protected or isolated from excessive shock, to provide the required life.
U.S. Pat. No. 4,871,495 of Helferich and Schenck, issued Oct. 3, 1989 and assigned to the same assignee as this invention, incorporated herein by reference, describes and claims a ceramic trap composition which is capable of being molded, such as by casting, injection molding, or extrusion, into a wide variety of shapes and configurations. The composition provides a ceramic trap material which has a predominantly open celled porosity. This material has significant advantages in an ability to manipulate the cell size and mass, as to provide a wide variety of characteristics in the final article. The moldable composition, after processing and firing, forms a ceramic microstructure characterized by low thermal expansion, and high thermal shock resistance with suitable refractoriness.
A further characteristic of the product and composition as described in the above-identified copending application resides in an ability to form, on one or more exposed surfaces, a "membrane" layer having a pore size which is substantiallY smaller than the pore size of the remaining body of the material. The membrane surface forms a final trap or filter when the material is used as an element in a trapping system. The product and composition as disclosed in the above mentioned Helferich et al patent have particular advantage when used as trapping elements in a trap containing a plurality of such elements arranged in parallel to each other with respect to the direction of exhaust gas flow. Therefore this product is particularly adapted for use in the elements of the side entry diesel particulate trap as described herein.